Prostheses for knee replacement have been developed to try to recreate the natural movement of a knee which has deteriorated due to injury or disease. The complex movement of the knee presents a great challenge to the inventor. Historically, knee prostheses have tried to address these different aspects of knee movement but usually to the detriment of one function or another. This difficulty in recreating natural knee movement is apparent from a description of relevant knee structure.
The knee is a joint between the lower part of the femur which is the upper bone of the leg, and the upper part of the tibia which is the lower bone of the leg. The lower part of the femur is rounded and offset from the longitudinal axes of the straight portion of the femur. The rounded end of the femur has two knuckle like protrusions called condyles. Fluoroscopic research has shown that the femoral condyles of the normal knee contact the tibia anterior to the midline of the saggital plane at knee full extension. As the knee flexes, both condyles move in a posterior direction. The lateral condyle however, moves much more posterior than the medial condyle. At full extension the lateral condyle contacts the tibia approximately 5 to 10 mm anterior. The medial condyle contacts the tibia from 0 to 3 mm anterior at full extension. As the knee bends, the lateral condyle rolls back to a position of 10 to 15 mm posterior at 120 degrees of knee flexion. The medial condyle however, moves back only 4 to 5 mm to a final position of 1 to 3 mm posterior. Therefore, since the lateral condyle is moving more posterior than the medial condyle, screwhome motion is allowed to occur. Screwhome motion is the relative rotation of the lateral condyle with respect to the medial condyle. In the normal knee, research has shown up to 15 degrees of screwhome motion during a deep knee bend. Research on the normal knee has also shown that the lateral condyle rolls back 10 to 15 mm during a deep knee bend.
This relative movement between the femoral condyles and the tibia is due in part to the fact that the condyles are offset from the longitudinal axis of the femur. However, that is not the only reason. This complex combination of rolling and sliding is defined by not only the configuration of the respective surfaces of the femur condyles and the tibia, but also by the structure of connecting tendons, ligaments and tissue. These connecting elements are partially or entirely removed in typical prostheses installations.
Further complicating the movement is the torsional effect previously mentioned, whereby the tibia turns about its longitudinal axis as it flexes or extends. It is well known that the tibia rotates outward (that is, the right tibia rotates clockwise and the left tibia rotates counterclockwise, as viewed from above) as the tibia extends. This produces a flaring out of the feet during extension and the reverse during flexion. This effect is a result of complex interaction at the knee joint. The absence of this effect in a prosthesis may produce discomfort and awkwardness in walking.
In summary, knee prostheses should replicate the range of movement of the knee from full extension to full flexion and back to extension again while recreating the simultaneous posterior rollback of the femoral condyles on the tibia during flexion as well as torsional movement. Several prostheses have been developed which attempt to recreate the natural shape and movement of the knee. These condylar prostheses, so named because they attempt to replicate the shape of the femoral condyles, have two components. The femoral component recreates the condylar shape of the lower femur with one end forming a groove in which the patella glides. The tibial component has recessed surfaces to receive the condylar portions of the femoral component. The tibial component also may have a spine protruding upwards into a slot in the femoral component which guides and restricts the range of movement of the joint through flexion and extension.
A successful example of a condylar prosthesis is disclosed in U.S. Pat. No. 4,298,992 granted to Burstein et al. That invention included a slot in the femoral component which engages the spine of the tibial component during flexion to recreate posterior femoral rollback. The slot in the Burstein et al. invention engages the spine at a point that is quite high on the spine. The resulting pressures on such a small point of contact require a large spine and sufficient bone resection to accommodate it.
The prostheses disclosed in U.S. Pat. Nos. 4,888,021 and 5,011,496 both granted to Forte et al. contain a spine on the tibial component which is shaped to provide a large contact surface area with the femoral component. The intent is to enlarge the area of contact between the components to reduce the wear resulting from high pressures on small areas of contact. The invention also provides for a separate patellar component intended to fit within the patellar groove portion of the femoral component.
U.S. Pat. No. 5,549,686 granted to Johnson et al. sought to improve upon the prior art by locating a tapered cam on the femoral component between and towards the posterior end of the condyles, that is, away from the end forming the patellar groove. The cam engages the spine of the tibial component at a lower point on the spine, thereby requiring a smaller spine and allowing for greater resistance to pressures because the spine is thicker towards its base. This configuration reduces the amount of bone to be resected.
Other knee prostheses are disclosed in U.S. Pat. No. 4,892,547 granted to Brown; U.S. Pat. No. 4,959,071 granted to Brown et al.; U.S. Pat. No. 5,219,362 granted to Tuke et al. and U.S. Pat No. 5,330,534 granted to Herrington et al.
Each of these developments in the prior art relies upon the sliding of the tibial component across the femoral component to reproduce the movement of the knee. While the femoral and tibial component engagements are designed to recreate posterior femoral rollback and to restrain the range of movement of the joint, they fail in accurately mimicking the natural movement of the knee joint for a variety of reasons. One strong reason is that they fail to emulate natural torsional movement. Over an extended time this may result in excessive wear on the components, as well as poor utility for the patient. Degradation of the component and the joint results in great discomfort and increasing disability. Ultimately, the prosthetic components may fail, necessitating expensive and intrusive revision or replacement surgery on the joint. Replacement of the prosthetic components require additional preparation of the remaining bone of the joint and repeat fixation of the prosthesis to the bone. The additional work on remaining bone structure may result in less effective bonding of the component and, thus, a weakening of the joint. Fluoroscopic research has shown that during a deep knee bend prior art protheses do not replicate normal posterior femoral rollback. Since the cam and post do not contact during gait, fluoroscopic studies have shown that such prostheses do not replicate normal knee motion. In fact, sliding often occurs.